The present invention relates to a system for drying web material and more particularly, to a drying system utilizing nozzles which can be removed and/or adjusted conveniently and quickly.
For nearly two centuries, the dominant system for drying continuous webs has been a series of rotating steam heated cylinders. When drying surface coatings on paper webs, the alternating contact of the web with the metal cylinder results in picking and other quality defects. This problem was a primary stimulus for the development of flotation drying systems that simultaneously support and convectively dry the web on a cushion of high velocity hot air. In the past twenty years, flotation drying has evolved into the dominant method for drying coated webs.
With a typical flotation web drying installation, the drying is accomplished by an array of nozzles positioned on each side of the web. Heated air is transported to the nozzles by a system of headers to which the air is supplied through a duct. A similar duct collects the air after it leaves the vicinity of the web.
There are various types of nozzles used for flotation drying. One known type of nozzle, which is taught by U.S. Pat. No. 3,587,177, the teachings of which are incorporated herein by reference, utilizes a "negative" pressure gas cushion to support and dry the web. U.S. Pat. No. 4,414,757 teaches another known type of nozzle utilizing a "positive" pressure to float and dry the web and the teachings of this patent are also incorporated herein by reference. This nozzle also imparts an undulating web flow characteristic which is preferable for most drying applications, and such nozzles display very uniform machine-direction heat transfer. Positive pressure nozzles are tolerant of a very large range of web tension, and they allow the web to resist the formation of edge curl.
In laboratory research, development work, and in actual practice, it is desirable to be able to quickly mount, locate and change the nozzle housings. Normally, however, mounting bolts for assembly and disassembly are located at both the front and back end of the nozzle housing. While removal of the front mounting bolts is easily accomplished, it is extremely difficult to reach the mounting bolts at the back of the nozzle housing. Assembly and disassembly is therefore hampered.
Moreover, when mounting bolts secure both the front and the back end of the nozzle housing, the nozzle housing expansion may be restricted upon heatup of the dryer. The nozzles may become bowed. The nozzles can bow toward the web or product being dried as the web passes through the gap between opposed nozzles. Consequently, the web may rub on the nozzles causing the web to tear and break resulting in shut down of the machine and loss of production time.
Accordingly, it is an object of the present invention to provide a nozzle assembly for use in flotation drying systems which can be quickly assembled and disassembled.
It is another object of the present invention to provide an improved apparatus and method to secure a nozzle housing to the supply bulkhead in a flotation drying system.
Still another object of the present invention to provide a nozzle assembly for flotation drying systems that facilitates nozzle expansion upon heatup of the dryer and therefore reduces the possibility of nozzle bowing.